The present invention relates to an ultrasonic sensor for controlling the process during resistance spot welding.
The essence of the method described in European Patent Application EP-A-653 061 is to investigate the intended weld region by ultrasonic transmission during the welding operation using shear and transversal waves by positioning an ultrasonic transmitter and an ultrasonic receiver for shear waves on each of the external electrode adapters of the two diametrically opposed welding electrodes. Starting at the ultrasonic transmitter on one welding electrode, the ultrasonic signal passes through the weld material—two or more sheets to be welded—and the other welding electrode until it reaches the ultrasonic receiver. Said ultrasonic receiver converts said ultrasonic signal to a measurable electrical signal U, the shape of the curve of which over time can be depicted using the equation U=UO·sinωt, where ω=2 πf is the angular frequency of the ultrasonic wave, f is the frequency, and t is the time. The through-transmitted signal is detected online, and its amplitude UO is used as the control variable for amplitude and the shape of the welding current curve over time. The transversal wave is selected because the influence of fluid formation in the weld nugget on the dampening of a through-transmitted wave is very strong with this type of wave. The amplitude UO of the transversal wave, which changes markedly and in characteristic fashion over the course of the welding process, permits a reliable determination of the formation and size of the weld nugget and can therefore be used as a manipulated variable for a control process.
The basic feasibility of the method and the reliability of the examination findings are crucially dependent on the ultrasonic sensors used, their location relative to the welding electrodes, and the sound propagation inside the welding electrodes. In the realization according to EP-A-653 061, an arrangement of ultrasonic sensors is selected in which the ultrasonic transmitter and ultrasonic receiver are mounted on the external electrode adapters or on the electrode holders, which are not shown in the drawing. Shear waves, transversal waves, or torsional waves having a frequency of less than 1 MHz are generated. It is stated that it is particularly advantageous to generate horizontally polarized transversal waves, since they have a low tendency to undergo undesired mode changes when reflections occur inside the sound-directing electroder holder. The ultrasonic transmitters and receivers are “shear wave test heads”. They contain flat and, usually, round piezoelectric plates having a diameter ranging from a few mm to a few cm, and that execute a shearing motion when excited with electric voltage or, conversely, when they receive, they react to a received shear wave with a reception voltage. Since, when a shear wave test head of this type is mounted directly on the external electrode adapter, the main emission direction of the sound would not be directed in the direction of the weld material, but rather at the center of the electrode, wedge-shaped attachments are preferably used, that are installed between the test heads and the welding electrodes and permit the main emission direction of the test head to be aligned with the weld material at an angle that is markedly less than 90°, e.g., approximately 45°. This arrangement does now allow all of the sound energy that is produced to be concentrated in the direction of the welding spot, however.
German Patent Application DE-A-199 37 479, which was published at a later date, describes an ultrasonic sensor system that is improved in this regard. With said ultrasonic sensor system, the piezoelectric shear wave plate or a complete shear wave test head is installed in a recess inside the electrode adapter for transmitting and/or receiving. In fact, said piezoelectric shear wave plate or the complete shear wave test head is installed in such a manner that the piezoelectric plate is oriented almost perpendicular to the electrode adapter, and the main emission direction of the transmitter and the main reception direction of the receiver are therefore parallel to the electrode adapter and are directed toward with each other. This allows all of the ultrasonic energy that is produced to be concentrated in the direction of the welding spot and, from there, it is transmitted in the direction of the receiver. This allows such a level of ultrasonic intensity to be produced in the welding spot and, during reception, it allows a received signal to be generated that is so great that a much greater wanted-to-unwanted signal ratio exists with regard for the further evaluation for controlling the welding process.
In the case of the latter sensor system, described in German Patent Application DE-A-199 37 479, in which the sensors are integrated in specially-configured welding electrodes and that is optimized with regard for the wanted-to-unwanted signal ratio of the ultrasonic signals, it is also considered disadvantageous that the electrode adapters typically used with spot welding can no longer be used. It was possible to manufacture said electrode adapters as a simple turned component at very low cost, due to their smooth cylindrical shape. With the sensor system according to German Patent Application DE-A-199 37 479, however, the complete welding electrode with integrated sensor must be replaced if mechanical damage occurs to the electrode adapter or if a sensor defect occurs.
With resistance spot welding, however, a large number of weld objects have different shapes, so a large number of welding electrodes having different shapes is used. Since the diameter of the welding electrodes is the same for most applications, the large variety of different types is due mainly to the welding electrodes having different lengths and the end pieces having different shapes, which said end pieces must be accommodated by corresponding electrode holders. There are also many different types of said electrode holders. For this reason, one must put up with a large variety of different types of welding electrodes having integrated sensors with regard for the acoustically optimized sensor system according to German Patent Application DE-A-199 37 479 as well.
With regard for the sensors mounted on the external electrode adapter, no statements are made in European Patent Application EP-A-653 061 about how the shear wave test heads, including the wedge-shaped attachments, are mounted on the external electrode adapter. This is a fundamental problem. Not only must the connection have good acoustic conductivity, it must also be so mechanically sound that the sensors and/or wedge-shaped attachments are able to withstand the high accelerations that occur when the welding tongs open and close during the welding process and not drop out of the welding electrodes when the welding electrodes impact the sheets to be welded. Since, in addition, considerable temperature fluctuations can occur at the welding electrodes, an adhesive connection is not suitable. This would also prevent easy replacement of the sensors.
The object of the present invention, therefore, is to provide an ultrasonic sensor for controlling the process during resistance spot welding, in which the sensors and test heads are oriented entirely in the direction of the welding spot and are mounted on the external adapter of preferably cylindrically shaped welding electrodes in such a manner that simple installation and replacement is enabled.